The dependency of the resolution of optical lithography on the wavelength and the numerical aperture has continuously been reflected by the further development of corresponding exposure tools. Due to its significantly shorter wavelength, EUV lithography at 13.5 nm (92 eV) is therefore a promising technology for the next generation lithography. It has been demonstrated that chemically amplified resists (CAR) can achieve a resolution of 25 nm half-pitch line:space features with interferometric EUV lithography (J. W. Thackeray, et al., Proc. SPIE 6517, 651719 (2007)). However, source power and resist performances are far from the set targets, thus leading to very slow photospeed. For example, the targeted source power for EUV manufacturing tools is 180 W delivered to the illuminator optics, while currently used sources struggle to reach even intensities as low as 10 W at intermediate focus. Similar to the tool, EUV resists also need significant improvements to reach the targeted performance. For example, the targeted dose for a resolution of 32 nm is 10 mJ/cm2 and the line width roughness is 1.7 nm. Currently investigated resist systems are far from these values. Due to the present issues with EUV source power, resist sensitivity comprises one of the main areas where improvement is required.
In chemically amplified resists (H. Ito, Adv. Polym, Sci. 2005, 172, 37; G. Wallraff, W. Hinsberg, Chem. Rev. 1999, 99, 1801), PAGs (J. Crivello, J. Polym. Sci. Part A: Polym. Chem. 1999, 37, 4241; H. Ito, C. G. Wilson, Polym. Eng. Sci. 23, 1012 (1983)) play a key role in imaging. In conventional optical lithography, CAR formulations contain a nearly transparent polymer matrix, a PAG and additional compounds, such as base quenchers, in smaller quantities. PAGs are non-acidic molecules that absorb photons and subsequently form acid via a photochemical decomposition. Acid is only formed in illuminated regions of the resists and only in these regions, the resist polymer becomes soluble (usually after baking) in basic developers.
Traditional, commercially available PAGs are for example triphenylsulfonium nonafluorobutanesulfonate (1; “TPS PFBuS”) or bis(4-tert-butyl-phenyl) iodonium nonafluorobutanesulfonate (2; “DTBPIO PFBuS”) having the following structure:

Some less commonly used, commercially available PAGs have less fluorine in the anion, such as TPS camphorsulfonate (3). However, the corresponding acid, camphorsulfonic acid, is not as strong as nonafluorobutanesulfonic acid. Moreover, the cation is still not very absorbing at 13.5 nm.
Li et al., U.S. Pat. No. 7,235,342, col. 14, line 23 to col. 15, line 2 disclose other typical PAGs used in the related art.
While the absorption of photons whose energy ranges from deep ultraviolet (˜150 nm) to near infrared (˜850 nm) by organic molecules, thus by PAGs, depends on the existence of certain chemical bonds, the absorption of EUV light is characterized by photoabsorption cross-sections that depend on the atomic composition of the compound which varies by atom (FIG. 1).
It is known that EUV photons are also absorbed by the resist polymers and other components and not just by the PAG. (P. Dentinger, et al., Proc. SPIE 3997, 588 (2000)). Under EUV irradiation, acid therefore may be generated by secondary processes as well and not only by direct hits of the PAG by the photons. (T. Kozawa, et al., Jpn. J. Appl. Phys. 31, 4301 (1992)) However, literature reports also indicate a strong PAG structure dependence on the acid generation efficiency in EUV light. (C. M. Szmanda, et al., J. Vac. Sci. Technol, B 17(6), 3356 (1999); T. Watanabe, et al., Jpn. J. Appl. Phys. 44, 5866 (2005))
Ionic PAGs have the general structure P+ A−, where P+ decomposes into protons (H+) upon irradiation with photons, while A− remains unchanged and forms the acid H+ A−. In an efficient PAG, P+ therefore strongly absorbs the photons of interest, while A− does not. In contrast, if A− absorbs too many photons, the acid generation efficiency may decrease because this decreases the amount of photons that could otherwise reach P+, or because A− may decompose as a consequence of photon absorption and therefore weakens the generated acid.
Commercially available PAGs, such as compounds 1, 2 and 3, commonly used in polymer formulations for optical lithography and presently used in EUV lithography, do not have an EUV-optimized atomic composition. For example, in the commonly used PAG triphenylsulfonium perfluorobutanesulfonate (1, TPS PFBUS), where P+ comprises the triphenylsulfonium cation and A− comprises the perfluorobutanesulfonate anion, the EUV photoabsorption cross-section of P+ is one order of magnitude lower than that of A− (FIG. 2). In the case of bis(t-butylphenyl)iodonium perfluorobutanesulfonate (2; DTBPIO PFBuS), the photoabsorption cross-section of P+ is still almost one order of magnitude lower than that of A−, though P+ contains the highly EUV-absorbing iodine atom. To overcompensate lack of optimum EUV sensitivity, resist formulations with much higher PAG loadings than typical for photolithographic applications are used. This can lead to increased line edge roughness and high resist outgassing.